Titian's Diana and Actaeon, the subject of a fundraising effort to be kept in the UK. Photograph: National Galleries of Scotland/PA

Before the new year, it is expected to be officially announced that Titian's Diana and Actaeon has been saved for the nation. The £50m price tag on the 500-year-old masterpiece will be reached through a mixture of grants from the government and Scottish parliament, fundraising by the National Gallery in London and the National Gallery of Scotland, and public and private grants and donations. Then the nightmare really starts for the gallery directors: they have to start fundraising all over again, not just for the companion Titian, Diana and Callisto, but the vast endangered collection in the shadows behind it, a treasury of private loans to public collections - which could at any time be taken off the walls and sent to the saleroom.

Conservative estimates suggest that one in 50 of all works in the national art collections is a loan - and as even the super-rich face a tough 2009, private owners may be moved to sell.

If all the art on private loan were assembled under one roof, its value would run into billions. As well as the Titians, the loans include the rest of the Bridgewater collection - wonderful paintings by Poussin, Rubens, Raphael, Rembrandt, Van Dyck, Monet, Manet, and Gauguin, bronzes and jades, tapestries and marble sculptures. No museum in the land, no possible combination of lottery funds, charity awards and government grants could pay for them all.

The National Gallery in London alone has almost 250 loans, the oldest dating to 1927. Most are from other museums or institutions, or collectors who will never sell, like the Queen's loan to the V&A of seven enormous tapestry designs by Raphael.

Temptation to sell

But many others are from private owners genuinely moved to share their treasures, or delighted to get rid of the security and insurance liabilities for a period. Many were tempted to sell in the now stalling art market boom, and the fear is that many more will be in the next few years.

The campaign to save the Titians has exposed the fatal flaw in Britain's system for saving works of art. There are tax breaks for giving money to public institutions, and once the collector is safely dead, works of art may be accepted in lieu of inheritance tax. But a living collector will have to be moved by pure philanthropy to give a work of art: despite the recommendations of the government-commissioned Goodison review, which has been gathering dust for almost five years, and unlike the position in many other countries including the US, there isn't a penny of tax concession.

Andrew Macdonald, deputy director of the Art Fund charity - invariably the first stop for museums desperate to keep a treasure - which has given £1m towards the Titian appeal, said: "The Art Fund has long argued that a change to the tax system to encourage lifetime gifts of works of art to museums and galleries could play a significant part in enabling galleries to develop their collections, and this is especially true when sources of funding over the next few years will inevitably be hard pressed."

Mark Jones, the director of the V&A - which is about to borrow back a magnificent Moghul jade flask once owned by Clive of India, on loan to the museum from his descendants for decades, then bought for a world record price by the royal family of Qatar - also believes reform is essential, and not just to help with fundraising.

"To me the real relationships with private collectors are made through gifts of art objects, rather than money - and so it doesn't seem to me very sensible that if you give a gift of £1m to a museum the tax concessions mean it only costs you £600,000, whereas if you give an object worth £1m it costs you the full amount."

The Duke of Sutherland's Titians are only a fraction of the Bridgewater collection, placed by his ancestor in the National Gallery of Scotland in Edinburgh in 1945. They are the core of that museum's collection, and the most valuable art loan ever made in Britain - which he now wishes to sell.

The £50m Diana and Actaeon comes with the offer for the same price of the sister painting of Diana and Callisto, and four more years to raise the price. And then he hopes an as yet unspecified sum will be raised for the rest of the Bridgewater collection - valued at £1bn by the Art Newspaper, including three works by Raphael, a Rembrandt, three more Titians and seven by Poussin.

The current Duke wants to see them in a public collection, and is offering them at a stupendous but bargain price - but the task of fundraising will be phenomenal.

Troubled acquisitions

Like Jones, every national museum director has borne the anguish of a treasure vanishing into an auction house. They hope the crisis over the Titians may finally prompt the government into tax concessions for living donors.

The National Gallery, which succeeded in buying Raphael's Madonna of the Pinks for £35m from the Duke of Northumberland four years ago after another epic campaign, lost another Titian, a portrait of an anonymous, handsome young man with an expression of scholarly melancholy. The Earl of Halifax failed to sell the painting, and it is now on temporary loan again to the Renaissance Faces exhibition.

Two years ago the gallery started fundraising for five Poussin paintings, The Sacraments, when the Duke of Rutland announced he was withdrawing the loan and selling at an estimated £100m. He then abandoned the sale, and the paintings are still in the gallery - but the loan is only guaranteed for another three years. The Louvre in Paris is reported to be very interested in his intentions.

Even the National Maritime Museum lost an eccentric treasure after a long loan, a vast, battle-scarred 18th-century flag, but managed to raise the price when the government imposed a temporary export bar - the very last resort for keeping an object in the national collection, but which adds enormously to the cost of acquisition.

The National Trust also has thousands of objects on loan to furnish its historic properties, often from the descendants of the original owners.

Its chief curator, David Adshead, said: "Visitors tend to assume all the objects they are admiring are safe, so it has proved very difficult to raise a fighting fund."

At the V&A, Jones thinks tax breaks could bring in pieces which would fill major gaps. "I maintain contacts with a circle of younger collectors - and they say they would like to give, but they just can't afford to."